Background paper on subsidence monitoring and measurement with a focus on coal seam gas (CSG) activities

Subsidence is the downward movement of the surface of the Earth relative to some predefined level and the monitoring of subsidence requires detecting changes in the Earths surface across a region over a period of time. An issue arising from Coal Seam Gas (CSG) activities is whether the extraction of gas and groundwater causes subsidence at the surface. The purpose of this background paper is to describe the measurement techniques that are available which might be used to monitor regions where CSG activities are either taking place already or are identified for future activities. The question, at its simplest level, amounts to defining the accuracy with which ground deformation can be measured and at what temporal and spatial scales. Both ground-based and space-based observations can be made to detect changes in the Earths surface. All require repeat measurements in order to derive estimates of change (i.e. subsidence). The more frequently the observing programme is repeated, the greater the insights will be in terms of how the subsidence pattern evolves over time. The most appropriate measuring technique for any particular CSG activity monitoring will depend upon the spatial extent of the expected deformation and the likely magnitude. Crude techniques can be used to detect large (i.e. > 0.5 m) deformation signals, while more sophisticated and more expensive techniques are required to detect small (i.e. < 1 cm) deformation signals. A potentially useful side benefit of a careful subsidence-monitoring programme could be that information can be obtained on the nature and propagation of the movement of gas and water at depth below the surface of the Earth. In essence, highly accurate knowledge of the movement of the surface can be used to derive estimates of pressure and volume changes occurring at depth as a result of CSG activities. This information may be of interest to both the mining companies and the Government by providing additional detailed information on how the CSG activities are progressing